Re: [PATCH 4/5] mm/swap_state: fix potential faulted in race in swap_ra_info()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/4/9 16:50, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> While we released the pte lock, somebody else might faulted in this pte.
>> So we should check whether it's swap pte first to guard against such race
>> or swp_type would be unexpected. And we can also avoid some unnecessary
>> readahead cpu cycles possibly.
>>
>> Fixes: ec560175c0b6 ("mm, swap: VMA based swap readahead")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/swap_state.c | 13 +++++++++----
>>  1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>> index 709c260d644a..3bf0d0c297bc 100644
>> --- a/mm/swap_state.c
>> +++ b/mm/swap_state.c
>> @@ -724,10 +724,10 @@ static void swap_ra_info(struct vm_fault *vmf,
>>  {
>>  	struct vm_area_struct *vma = vmf->vma;
>>  	unsigned long ra_val;
>> -	swp_entry_t entry;
>> +	swp_entry_t swap_entry;
>>  	unsigned long faddr, pfn, fpfn;
>>  	unsigned long start, end;
>> -	pte_t *pte, *orig_pte;
>> +	pte_t *pte, *orig_pte, entry;
>>  	unsigned int max_win, hits, prev_win, win, left;
>>  #ifndef CONFIG_64BIT
>>  	pte_t *tpte;
>> @@ -742,8 +742,13 @@ static void swap_ra_info(struct vm_fault *vmf,
>>  
>>  	faddr = vmf->address;
>>  	orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
>> -	entry = pte_to_swp_entry(*pte);
>> -	if ((unlikely(non_swap_entry(entry)))) {
>> +	entry = *pte;
>> +	if (unlikely(!is_swap_pte(entry))) {
>> +		pte_unmap(orig_pte);
>> +		return;
>> +	}
>> +	swap_entry = pte_to_swp_entry(entry);
>> +	if ((unlikely(non_swap_entry(swap_entry)))) {
>>  		pte_unmap(orig_pte);
>>  		return;
>>  	}
> 
> This isn't a real issue.  entry or swap_entry isn't used in this

Agree. It seems the entry or swap_entry here is just used for check whether
pte is still valid swap_entry.

> function.  And we have enough checking when we really operate the PTE
> entries later.  But I admit it's confusing.  So I suggest to just remove
> the checking.  We will check it when necessary.

Sounds reasonable. Will do it in v2.

Many thanks for review and reply!

> 
> Best Regards,
> Huang, Ying
> .
>